Synopsis

sd_bus_message_get_cookie() returns the
transaction cookie of a message. The cookie uniquely identifies a
message within each bus peer, but is not globally unique. It is
assigned when a message is sent.

sd_bus_message_get_reply_cookie()
returns the transaction cookie of the message the specified
message is a response to. When a reply message is generated for a
method call message, its cookie is copied over into this field.
Note that while every message that is transferred is identified by
a cookie, only response messages carry a reply cookie
field.

Both functions take a message object as first parameter and
a place to store the 64-bit cookie in.

No cookie has been assigned to this message.
This either indicates that the message has not been sent yet
and hence has no cookie assigned, or that the message is not a
method response message and hence carries a reply cookie
field.